Triple Axis Analyzer and Reciprocal Space Mapping

When measuring Omega-2Theta scans in double axis configuration, there is still a component in the scan due to scattering within the plane, as a simple slit allows this scatter into the detector . This is reduced as the slit is reduced. However, as the size of the slit is reduced the intensity is also reduced. To reduce this to acceptable acceptance angles, often a slit of 0.1mm is required, reducing the intensity by up to 95%. Clearly this is not the best solution to obtain a high resolution at the detector.

ingaas scan

Above is a double axis Omega-2Theta scan of a GaAs substrate with a graded InxGa1-xAs buffer layer. The tallest peak is the substrate peak, while the hump trailing off on the left is the buffer layer. The sharp peak on the right was a surprise. In this case, we did not know whether this peak was due to an additional layer with a different lattice parameter, a relaxed layer or from a tilted portion of the material.

ingaas rsm

The chart above is the triple axis map, plotted in diffractometer coordinates. The horizontal axis represents d-spacing changes (w-2q), while the vertical axis is mosaic tilt (w). The tall peak at (0,0) (in red) is the substrate peak. There is also another tall peak that has the same d-spacing as the substrate, but is shifted in the omega direction. This was the sharp peak that appeared on the right in the double-axis scan. From the triple axis map, we can determine that the sharp peak on the right is due to a tilted portion of the substrate, since the d-spacing matches exactly that of the substrate peak, while the peak is from a portion of material tilted with respect to the bulk of the substrate. The layer to the left of the substrate peak also changes in w as well as w-2q. This indicates the compositionally graded layer is also tilted, with the epilayer tilt increasing with misfit.